The invention relates to a shaft and sealing ring, the sealing ring having, more particularly, a cold flexed seal disk, and a method of its use for sealing the crankshaft of a two-cycle engine.
A known seal ring is produced from an annular disk of a non-elastomeric plastic, preferably polytetrafluoroethylene (PTFE). The outer circumference of the disk is in a stiffening ring and the inner circumference of the disk is rolled in the axial direction so as to form a kind of cuff which engages the shaft under bias.
German Federal patent publication OS No. 25 53 290 describes a sealing ring of the above-described kind. It has a seal element produced from a ribbed but otherwise planar, annular disk. The seal element is rolled at its inner circumference toward the sealed chamber to form a cuff-like end section with its ribs contacting the shaft for pumping any leakage thereat back to the sealed space. The attainment of good sealing action depends, for this reason, on the shaft's turning at the correct speed.
If the circumferential speed of the shaft is too great, intensified pumping action of the ribs toward the sealed space results. This can cause lubrication failure around the dynamic sealing zone of the ribs, as well as the pumping of dust from the surroundings into the dynamic sealing zone and even into the sealed chamber. All of these events are undesirable and can result in the premature failure of the seal.
If the circumferential speed of the shaft is too little, decreased pumping action of the ribs toward the sealed space results. Below a certain threshold velocity, therefore, the dynamic sealing zone can shift toward the outside, epsecially at the interstices between individual ribs, to allow leakage or contamination. Particularly when the shaft is at rest or vacuum builds up in the sealed space, these effects are plain to see. Both are undesirable.
It is not possible, therefore, to use such a seal for sealing the crankshaft of a two-cyle engine in its crankcase, because vacuum is produced in its crankcase. The outside air and the contaminants it contains then easily pass through the dynamic sealing zone of the seal into the "sealed" crankcase. The presence of the ribs intensifies this problem.